Iodine is used for roentgen contrast agent, drug, germicide or fungicide as a raw material for daily lives, or for catalyst stabilizer, photographic material as an industrial use, and also for feed additive, herbicide, etc. as an agricultural use, and is a precious resource on a global basis. Further, in recent years, it is used, for example, in a manufacturing step of liquid crystal films, and has become an extremely important resource indispensable in the main industries in Japan. However, production of iodine is limited mainly to two countries of Japan (centered at Kanto district, producing about 40% of the world production) and Chile in South America, making iodine an extremely limited precious resource. Accordingly, recovery of expensive iodine from used iodine-containing materials containing iodine or iodine compound after utilized for specific use in each industry such as the aforementioned raw materials for daily lives, industrial chemicals, agricultural chemicals, and further, manufacturing step of the liquid crystal films, is very beneficial from the viewpoints of economy, natural environmental conservation and natural resources saving. As described above, iodine is used for raw materials, intermediates, catalysts, etc. for various products. For example, iodine is used for synthesis of an organic compound as an elemental substance or a compound, but the whole iodine used is not necessarily involved in the synthesis. Some parts may remain unreacted, or be converted to another compound. Also, when a solvent is used in a purification step to improve purity of a synthesized product, the entire product is not necessarily recovered simply, because some parts of the product may be dissolved in the solvent. Accordingly, iodine-containing materials discharged from production processes contain iodine in a form of an elemental substance or various organic or inorganic compounds, and its state also varies, for example, like waste liquid, waste oil and sludge. In the past, regarding to recovery of iodine, various proposals has been made. In particular, as an iodine recovery method associated with combustion process, for example, a method has been known, in which iodine-containing material containing iodine or iodine compound is continuously fed into a combustion furnace to generate free iodine, and the iodine gas is absorbed by a sodium thiosulfate or sodium sulfite aqueous solution. Also, an iodine recovery roasting furnace has been known, which is used for recovering iodine from iodine-containing material containing an organic compound, iodine and/or an iodine compound and an alkali metal compound. These conventional methods are very beneficial processes in terms of recovering iodine from iodine-containing material.
However, in such conventional iodine recovering method or iodine recovering roasting furnace, such problems have been pointed out that an excessive amount of chemical agent is used and that operation is complicated.
Consequently, as a method to solve these problems, a method for recovering iodine has been proposed in JP-A-1994-157005, which is characterized by comprising mixing iodine-containing material containing iodine or iodine compound with an alkali metal compound and a solvent, introducing the mixture into a combustion furnace having a combustion equipment to give heat treatment, and absorbing an iodine compound contained in the heat treatment gas with an alkaline aqueous solution.
However, in the method described in the above JP-A-1994-157005, before introducing into a combustion furnace, it was necessary that the iodine-containing material containing iodine or iodine compound was mixed with an alkali metal compound and a solvent in advance, and the resultant mixture was then introduced into a combustion furnace. Therefore, there was a problem that in the mixing step, the iodine-containing material containing iodine or iodine compound reacted with the alkali metal compound to form an insoluble precipitate, which caused clogging of piping and nozzles. If the method dared to be carried out, a step to separate solid and liquid was required. In addition, in order to remove heat of neutralization generated by the neutralization reaction between an acid contained in the iodine-containing material and the alkali metal compound as a base, an apparatus was needed. In addition, there was such a problem that in order to mix the iodine-containing material containing iodine or iodine compound with an alkali metal compound and a solvent by finely adjusting an amount of each component to be charged so that prescribed pH was obtained, a complicated and advanced mixing and stirring apparatus equipped with a flow rate regulating mechanism and a pH regulating mechanism was newly required. In addition, for performing continuous treatment, a plurality of such mixing apparatuses had to be installed, and hence had to make this system furthermore complicated and advanced one. In addition, since there was no solvent suitable for both of waste liquid and waste oil containing iodine or iodine compound, it was very difficult to homogeneously dissolve an alkali metal compound in both of waste liquid and waste oil, and this caused such a problem that when both were subjected to the heat treatment, iodine sublimed without being immobilized and reaction efficiency and consequently recovery rate was reduced. In addition, if much iodine sublimed, an absorption tower in the following step had more loading, and conventional facility could not correspond. If this problem was left unsolved, iodine was emitted to the atmosphere and environmental pollution could be caused. In order to correspond to a heavier loading, a large size of absorption tower had to be installed, but this is not economical.